This invention relates to optically clear switch assemblies of the type which are used with display devices.
Membrane switches of the type comprising laminated sheets of suitable film having narrow hairline conductors on their surfaces are widely used and commonly available. Typically, a switch of this type will comprise a substrate having one conductor thereon, a separator or spacer having an opening therein, and a flexible film or diaphragm which also has a conductor thereon. The conductors on the substrate and on the diaphragm cross each other at the opening in the separator so that the switch can be closed by pressing on the diaphragm at the switch site. Polyester film such as Mylar(polyester film) is commonly used for the diaphragm and may also be used for the substrate and the separator although other materials such as filled epoxy are also used for the separator and the substrate.
Ordinarily, membrane switches are placed at a location which is remote from the display area of a device, such as a hand held calculator, and the optical characteristics of the switch assembly are not critical or even important. In fact, most previously known switch assemblies are opaque and their ability to transmit light is of no importance. Typical membrane type switches are shown in U.S. Pat. Nos. 3,308,253, 3,617,660, and 3,600,528.
Electronicsare now being built which have the switch assembly placed over the display area of the device; see Electronicas Nov. 11, 1976, page 27 wherein a hand held terminal is described which has a switch assembly overlying a terminal display so that the user need not take his eyes from the display during the interval between the time he presses selected switches on the terminal and observes the information which appears on the display. In devices of this type, the light transmission characteristics of the switch assembly become critical and a switch assembly having inferior or limited light transmission characteristics would not be suitable. Furthermore, it is desirable to provide a suitable filter on devices having a switch means in overlaying relationship to a display area in order that the information displayed will not be overwhelmed by reflected light and many of the previously used membrane switches have optical characteristics which are incompatible with the use of a suitable filter. Polyester has been used for transparent membrane switch assemblies but its light transmission characteristics are relatively poor and it is, furthermore, incompatible with a polarizing filter.
The instant invention is directed to the achievement of an optically clear switch assembly comprised of laminated sheets of cellulose acetate material such as cellulose tri-acetate. In accordance with the teachings of the instant invention, hairline conductors can be provided on surfaces of cellulose acetate material without chemical or dimensional degradation of the film and excellent adhesion of the conductors to the film is obtained. In general, the process by which the conductors are produced on the surface of the cellulose acetate film involves steps of providing a thin blanket coating of conductive metal on the surface by an electroless deposition process. The blanket coating is then masked with photosensitive resist material, exposed along lines which define the desired conductors, developed and the masking material is removed along the conductor lines. The conductors are then electroplated to the desired thickness, the mask is removed, and the blanket plating is selectively etched away. As will be described below, the complete process requires careful execution of several of the steps of the process to prevent chemical attack of cellulose tri-acetate film which would result in degradation of its physical and optical properties.
It is accordingly an object of the invention to provide an optically clear switch assembly. A further object is to provide a membrane switch assembly having cellulose tri-acetate film laminates. A further object is to provide a method of manufacturing an optically clear membrane switch assembly. A further object is to provide an improved optically clear switch which is shielded.